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Abstract

The terahertz (THz) frequency quantum cascade laser (QCL) is a compact source of high-power radiation with a narrow intrinsic linewidth. As such, THz QCLs are extremely promising sources for applications including high-resolution spectroscopy, heterodyne detection, and coherent imaging. We exploit the remarkable phase-stability of THz QCLs to create a coherent swept-frequency delayed self-homodyning method for both imaging and materials analysis, using laser feedback interferometry. Using our scheme we obtain amplitude-like and phase-like images with minimal signal processing. We determine the physical relationship between the operating parameters of the laser under feedback and the complex refractive index of the target and demonstrate that this coherent detection method enables extraction of complex refractive indices with high accuracy. This establishes an ultimately compact and easy-to-implement THz imaging and materials analysis system, in which the local oscillator, mixer, and detector are all combined into a single laser.

Figures (3)

Schematic diagram showing an intensity, and frequency modulated, self-mixing interferometer with representative waveforms in the time domain. (a) A laser with an external target at distance Lext from the exit facet of the laser. (b) Self-mixing signal observed through variation in laser terminal voltage as a function of time. Broken line: reference slope. Solid line: typical voltage signal. (c) Single period T of the self-mixing signal showing effect of increasing refractive index n of the target. The waveform narrows with increasing n, and the peak shifts to right. (d) Effect of increasing extinction coefficient k of the target. This mainly translates the waveform to the right.

Schematic diagram of the experimental apparatus used for target measurements. (a) Current stimulus signal; the current range was selected to sweep the laser frequency through three external cavity resonances in the region where the laser was most sensitive to optical feedback. (b) Corresponding voltage signal measured across the laser terminals. For illustrative purposes, the magnitude of the self-mixing signal has been increased ten-fold. The voltage signal shown is typical for this set-up. (c) The QCL is driven by a sawtooth current signal and the QCL terminal voltage variations are acquired using a PC-based data acquisition card (PC DAQ). A pair of parabolic mirrors focusses the beam onto the remote target containing materials under test, mounted on a computer-controlled translation stage. The six-dot triangular pattern was used for target orientation.

Targets, images, and exemplar time-domain traces. (a) Photograph of the front surface of the target. The three circular regions are materials under test embedded in an aluminium holder. Yellow — PA6. Green — PVC. Red — POM. Representative purple line of raster scan across PVC and POM is elaborated upon in (b). (b) Two-dimensional representation of time domain self-mixing signals acquired along representative line in (a), each showing three fringes. The vertical axis represents the temporal evolution of the signal whilst horizontal axis shows their spatial dependence. (c) Amplitude-like Image: Pseudo-colour plot representing the effective aggregate difference between the time domain trace relative to the reference slope. (d) Phase-like Image: Pseudo-colour plot based on the temporal location of the representative peak of the self-mixing signal relative to the edge of the modulation waveform. (e) Representative time domain waveforms for one spatial pixel on the target per material (solid lines) with corresponding model fits (broken lines). The common reference slope is removed.